(combine_bytes): New function.

(combine_bytes): New function.
(insert_1_both, insert_from_string_1, insert_from_buffer_1):
Simplify greatly by using combine_bytes near the end
to handle all aspects of combining except for undo-recording.
This means most of the code deals with the text as if there were
no combining.
(replace_range): Likewise.  Also handle unibyte-multibyte conversion
as in insert_from_string_1.
(del_range_2): Handle combining of character before the deletion
with strays after the deletion.
(adjust_markers_for_delete): Delete debugging code for Z == Z_BYTE.
(adjust_markers_for_insert): Move Z == Z_BYTE debugging code
to before we relocate the marker.